The "greenhouse effect" refers to the natural phenomenon that keeps the Earth in a temperature range that allows life to flourish.

The sun's enormous energy warms the Earth's surface and its atmosphere.

As this energy radiates back toward space as heat, a portion is absorbed by a delicate balance of heat-trapping gases in the atmosphereamong them carbon dioxide and methanewhich creates an insulating layer.

With the temperature control of the greenhouse effect, the Earth has an average surface temperature of 59°F (15°C). Without it, the average surface temperature would be 0°F (-18°C), a temperature so low that the Earth would be frozen and could not sustain life.

"Global warming" refers to the rise in the Earth's temperature resulting from an increase in heat-trapping gases in the atmosphere.

Scientists have concluded that human activities are contributing to global warming by adding large amounts of heat-trapping gases to the atmosphere.

Our fossil fuel use is the main source of these gases.

Every time we drive a car, use electricity from coal-fired power plants, or heat our homes with oil or natural gas, we release carbon dioxide and other heat-trapping gases into the air.

The second most important source of greenhouse gases is deforestation, mainly in the tropics, and other land-use changes.

Since pre-industrial times, the atmospheric concentration of carbon dioxide has increased by 31 percent. Over the same period, atmospheric methane has risen by 151 percent, mostly from agricultural activities like growing rice and raising cattle.

As the concentration of these gases grows, more heat is trapped by the atmosphere and less escapes back into space. This increase in trapped heat changes the climate, causing altered weather patterns that can bring unusually intense precipitation or dry spells and more severe storms.

In 1988, the United Nations Environment Programme and the World Meteorological Organization set up the Intergovernmental Panel on Climate Change (IPCC) to examine the most current scientific information on global warming and climate change. More than 2,500 of the world's leading climate scientists, economists, and risk experts contributed to the panel's most recent report, Climate Change 2001: The Third Assessment Report.

Scientists from about 100 countries were involved in this new reportmore than in any previous report and with greater participation from developing countries. These scientists reviewed all the published and peer-reviewed scientific information produced during the previous few years to assess what is known about the global climate, why and how it changes, what it will mean for people and the environment, and what can be done about it.

The Third Assessment Report is the most comprehensive and up-to-date evaluation of global warming. As the new benchmark, it serves as the basis for international climate negotiations.

Yes. The Intergovernmental Panel on Climate Change concluded in its Third Assessment Report, "An increasing body of observations gives a collective picture of a warming world and other changes in the climate system." The kinds of changes already observed that create this consistent picture include the
following.

Examples of observed climatic changes:

Increase in global average surface temperature of about 1 degree Fahrenheit in the 20th century

Decrease of snow cover, sea-ice ice extent and the retreat of mountain glaciers in the latter half of the 20th century

Rise in global average sea level and the increase in ocean water temperatures

Likely increase in average
precipitation over the mid- and high-latitudes of the Northern
Hemisphere, and over tropical land areas

Increase in the frequency of extreme precipitation events in some regions of the world

In 1995, the world's climate experts in the Intergovernmental Panel on Climate Change concluded in a cautious consensus, "The balance of evidence suggests that there is a discernible human influence on the global climate."

In its 2001 assessment, the IPCC strengthened that conclusion considerably saying, "There is new and stronger evidence that most of the warming observed over the last 50 years is attributable to human activities."

Scientists have found significant evidence that lead to this conclusion:

The observed warming over the past 100 years is unlikely to be due to natural causes alone; it was unusual even in the context of the last 1,000 years.

There are better techniques to detect climatic changes and attribute them to different causes.

Simulations of the climate response to natural causes (sun, volcanoes etc.) over the second half of the 20th century alone cannot explain the observed trends.

Most model simulations that take into account greenhouse gas emissions and sulphate aerosols (which have a cooling effect) are consistent with observations over the last 50 years.

The Third Assessment Report from the Intergovernmental Panel on Climate Change projects that the Earth's average surface temperature will increase between 2.5 and 10.4°F (1.4 to 5.8°C) between 1990 and 2100 if no major efforts are undertaken to reduce the emissions of greenhouse gases (the "business-as-usual" scenario). This is significantly higher than what the Panel predicted in 1995 (1.8-6.3°F or 1.0-3.5°C), mostly because scientists expect a reduced cooling effect from tiny particles (aerosols) in the atmosphere.

Scientists predict that even if we stopped emitting heat-trapping gases immediately, the climate would not stabilize for many decades because the gases we have already released into the atmosphere will stay there for years or even centuries. So while the warming may be lower or at a slower rate than predicted if we reduce emissions significantly, global temperatures cannot quickly return to today's averages. And the faster and more the Earth warms, the greater are the chances for some irreversible climate changes.

Would a temperature rise of a couple degrees really change the global climate?

An increase of a few degrees won't simply make for pleasantly warmer temperatures around the globe. Even a modest rise of 2° to 3°F (1.1-1.7°C) could have dramatic effects. In the last 10,000 years, the Earth's average temperature hasn't varied by more than 1.8°F (1.0°C). Temperatures of only 5° to 9°F cooler than those today prevailed at the end of the last Ice Age, in which the Northeast US was covered by more than 3,000 feet of ice.

Scientists predict that continued global warming on the order of 2.5-10.4 degrees Fahrenheit over the next 100 years (as projected in the IPCC's Third Assessment Report) is likely to result in:

a rise in sea level between 3.5 and 34.6 in. (9-88cm) leading to more coastal erosion, flooding during storms and permanent inundation

severe stress on many forests, wetlands, alpine regions, and other natural ecosystems

greater threats to human health as mosquitoes and other disease-carrying insects and rodents spread diseases over larger geographical regions

disruption of agriculture in some parts of the world due to increased temperature, water stress and sea-level rise in low-lying areas such as Bangladesh or the Mississippi River delta.

Is global warming connected to the hole in the ozone layer?

NASA image  Ozone layer hole

Global warming and ozone depletion are two separate but related threats. Global warming and the greenhouse effect refer to the warming of the lower part of the atmosphere (also known as the troposphere) due to increasing concentrations of heat-trapping gases.

By contrast, the ozone hole refers to the loss of ozone in the upper part of the atmosphere, the stratosphere.

This is of serious concern because stratospheric ozone blocks incoming ultraviolet radiation from the sun, some of which is harmful to plants, animals and humans.

The two problems are related in a number of ways, including:

Some human-made gases, called chlorofluorocarbons, trap heat and destroy the ozone layer. Currently, these gases are responsible for less than 10 percent of total atmospheric warming, far less than the contribution from the main greenhouse gas, carbon dioxide.

The ozone layer traps heat, so if it gets destroyed, the upper atmosphere actually cools, thereby offsetting part of the warming effect of other heat-trapping gases. But that's no reason to rejoice: the cooling of the upper layers of the atmosphere can produce changes in the climate that affect weather patterns in the higher latitudes.

And finally, trapping heat in the lower parts of the atmosphere allows less heat to escape into space and leads to cooling of the upper part of the atmosphere. The colder it gets, the greater the destruction of the protective ozone layer.

Reducing ozone-depleting gases is crucial to preventing further destruction of the ozone layer, but eliminating these gases alone will not solve the global warming problem. On the other hand, efforts to reduce all types of emissions to limit global warming will also be good for the recovery of the ozone
layer.

Yes! The most important action we can take to slow global warming is to reduce emissions of heat-trapping gases. Governments, individuals, and businesses can all help.

Governments can adopt a range of options for reducing greenhouse gas emissions, including

increasing energy efficiency standards

encouraging the use of renewable energy sources (such as wind and solar power)

eliminating subsidies that encourage the use of coal and oil by making them artificially cheap

protecting and restoring forests, which serve as important storehouses of carbon

Individuals can reduce the need for fossil fuels and often save money by

driving less and driving more-fuel-efficient and less-polluting cars

using energy-efficient appliances

insulating our homes

using less electricity in general

Businesses can increase efficiency and save substantial sums by doing the same things on a larger scale. And utilities can avoid building expensive new power plants by encouraging and helping customers to adopt efficiency measures.

Reducing our impact on the global climate does not have to hurt the world's economies. The answer depends much on the "how" and "when."

The challenge is to strike a balance between responding early enough to avoid major negative (costly) impacts, and responding some time later in order to avoid taking big, expensive steps now which then may turn out to be unnecessary or inappropriate. This type of challenge is typical in business and industry  decision-making under uncertainty is the daily bread of most managers.

Clearly, global warming still involves many unknowns, but the remaining uncertainties in our scientific understanding no longer warrant a "wait-and-see" stance. Science tells us with increasing certainty that we are in for a serious long-term problem that will affect all of us.

And there is much we can do now that makes sense in terms of the economic bottom line and that helps reduce our impact on the global climate and on our local environment and health. America and other developed countries should seize the opportunity to take the lead in developing clean, energy efficient and new technologies, and help developing countries take a greener path to economic prosperity. All of this can be done in a cost-effective manner, while creating jobs and new business opportunities.

Why is there a difference between the surface and upper atmosphere temperature records?

Much debate in the scientific community has centered on the apparent discrepancy between the warming of temperatures recorded at the Earth surface and the much smaller rise in temperatures in the upper atmosphere. Climate contrarians have used this difference to say that global warming isn't real.

A recent report published by the National Academy of Sciences entitled "Reconciling Observations of Global Temperature Change" (2000) put this debate basically to rest. The National Academy of Sciences Panel of experts concluded:

"The warming trend in global-mean surface temperature observations during the past 20 years is undoubtedly real and is substantially greater than the average rate of warming during the twentieth century. The disparity between surface and upper air trends in no way invalidates the conclusions that surface temperature has been rising...."

The NAS Panel explained further that the smaller warming of the upper air is due to natural causes and human activities, processes that together go a long way in explaining why the actually observed upper air warming is less than what computer models predict.

Scientists base their evaluation of climate trends on much more than computer models and temperature records. Historical studies of glacier ice cores, tree rings, and coral growth layers have produced estimates of temperature and precipitation over much of the globe for the last 1,000 years. All these analyses show that the twentieth century is warmer than at any time in the last 1,000 years.

More questions?

If you have other questions about global warming, check our briefings, updates, recommendations, analyses, guides and links.

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